1. Field of the Invention
The present invention relates to an electric power steering apparatus using a motor as a generating source of steering assistance force and a method of manufacturing a gear used for the electric power steering apparatus.
2. Description of Related Art
An automobile is steered by transmitting rotating operation of a steering wheel disposed at the interior of passenger""s room to a steering mechanism disposed at the exterior of the room for turning tire wheels for steering (front wheels, in general).
FIG. 1 is a cross-sectional view showing a constitution of a conventional electric power steering apparatus.
The figure shows an example of an electric power steering apparatus for an automobile, comprising: a first steering shaft 101 connected to a steering wheel 100 for steering; a second steering shaft 103 connected at one end thereof via a torsion bar 102 to said steering shaft 101 coaxially and at the other end thereof to a steering mechanism joined to tire wheels; a torque sensor 104 for detecting a torque applied on the first steering shaft 101 with rotation of the steering wheel 100 based on torsion generated on the torsion bar 102; a steering assist motor 105 to be driven based on the result of detection of said torque sensor 104; and a reduction mechanism joined to an output shaft of said motor 105 for reducing rotation of said output shaft and transmitting the reduced rotation to the second steering shaft 103. Said reduction mechanism has a worm 106 and a worm wheel 107. The electric power steering apparatus is constituted to lighten driver""s load for steering by assisting the action of the steering mechanism corresponding to rotation of the steering wheel 100 with rotation of the motor 105.
The worm 106 constituting the reduction mechanism is supported at the interior of a housing 108 via a pair of antifriction bearings (not illustrated). The second steering shaft 103 provided with the worm wheel 107 is supported at the interior of the housing 108 via a pair of antifriction bearings 109, 109.
The worm wheel 107 comprises a ring-shaped tooth body 110 made of synthetic resin which has teeth meshing with the worm 106 and a support body 111 made of metal engaged with the interior side of said tooth body 110. The tooth body 110 made of synthetic resin reduces a jarring noise made by meshing with the worm 106 and realizes high workability of the teeth. A plurality of locking grooves 114 (see FIG. 2) are provided on a peripheral surface of the support body 111, and prevents relative rotation of the tooth body 110 and the support body 111.
FIG. 2 is a cross-sectional view showing a constitution of a conventional support body. The support body 111 of the worm wheel 107 is formed through cut processing as in FIG. 1 or through cold forging as in FIG. 2. The support body 111 formed through cold forging comprises: a cylindrical part 112 engaged with the tooth body 110; a plate part 113 extended inward in a radial direction from one end of said cylindrical part 112; and the locking grooves 114 provided on the peripheral surface of the cylindrical part 112. The support body 111 is integrally united with the tooth body 110 by placing the support body 111 in a mold for injection molding and performing injection molding.
However, when the support body is formed through cut processing and cold forging, there arises a problem that unnecessary deposit cannot be removed enough. The support body with great weight enlarges inertial force at the time of steering, and causes a deterioration of feeling of steering. In addition, cut processing and cold forging were disadvantageous in comparison with press forming from the viewpoint of manufacturing costs.
The present applicant has applied an electric power steering apparatus a support body of which is press-formed (Japanese Patent Application Laid-Open No. 2001-206230).
The engagement part of the press-formed support body with the tooth body comprises a disc part having a plurality of through holes to be filled with synthetic resin for molding the tooth body which penetrate in an axial direction and a curved edge part provided with a plurality of locking grooves which is curved in an axial direction from an exterior edge of said disc part.
A remedy has been desired for solving the following problems of the press-formed support body. One problem is that the support body is complicated in structure since a relative slide of the tooth body and the support body is hindered at two positions which are the through holes and the curved edge of the engagement part. Another problem is that circularity of synthetic resin in the mold for injection molding at the time of molding the tooth body is low since the synthetic resin for molding the tooth body is filled into the through holes of the engagement part, which results in generation of voids and weld marks.
In order to simplify a structure of the engagement part of the press-formed support body, a support body without the through holes is conceivable. Such a support body should have a curved edge which is long in an axial direction provided with locking grooves on the peripheral surface thereof to ensure an enough contact area with the tooth body. However, manufacture of said support body requires use of a relatively expensive and large presser to perform plastic deformation exclusively on the locking groove part on the peripheral surface of the support body. Consequently an amortization expense of the presser is added to a unit price of the support body, and the unit price of the support body amounts to approximately the same as that of the support body provided with the through holes.
The present invention has been made to settle the problematic points as described above. One object of the invention is to provide an electric power steering apparatus with which resin can be deposited uniformly, costs can be reduced, and a corrugated cylinder part can be engaged with a tooth body without generating voids and weld marks. Said electric power steering apparatus is constituted with the tooth body made of synthetic resin engaged with the exterior side of the corrugated cylinder part, said corrugated cylinder part being press-formed to deform the support body made of metal engaged with the interior side of said tooth body into a corrugated form corrugating in a peripheral direction.
Another object of the invention is to provide a method of manufacturing a gear with which a corrugated cylinder part with relatively long concavities can be provided simply by forming the relatively long concavities of the corrugated part with a relatively small presser. With said manufacturing method, the corrugated part configured as a corrugated plate is provided by performing press forming on a peripheral side of a metal plate circularly and the corrugated cylinder part is provided by bending said corrugated part into a cylindrical form with respect to a plate part. In the next place, the tooth body made of synthetic resin is integrally molded on the exterior side of said corrugated cylinder part.
The electric power steering apparatus according to the present invention is constituted to assist steering by transmitting rotation of a steering assist motor to a steering mechanism via a driving gear and a driven gear which has a ring-shaped tooth body made of synthetic resin meshing with said driving gear and a support body made of metal engaged with the interior side of said tooth body. The support body of the electric power steering apparatus comprises a corrugated cylinder part press-formed to have a plurality of concavities on the exterior side thereof and convexities to the interior side formed by said concavities which are arranged in a peripheral direction and a plate part an exterior edge of which is united with an end of said corrugated cylinder part.
With this constitution, unnecessary deposit on the support body can be removed satisfactorily in comparison with a conventional support body formed through cut processing and cold forging and resin can be deposited uniformly since the engagement side of the support body with the tooth body is configured as the press-formed corrugated cylinder part. In addition, the corrugated cylinder part which is relatively long in an axial direction and can ensure an enough contact area with the tooth body can be formed simply by press forming in comparison with the above-mentioned support body provided with the through holes and the curved edge part at the engagement part. Consequently manufacturing costs of the support body and the electric power steering apparatus using said support body can be reduced all the more. Further, synthetic resin for molding the tooth body can appropriately circulate along the concavities on the exterior side of the corrugated cylinder part since the corrugated cylinder part is engaged with the interior side of the tooth body, and the driven gear without voids and weld marks and the electric power steering apparatus using said driven gear are obtained.
The gear manufactured in the method according to the present invention is fitted to a transmitting shaft for transmitting rotation of the steering assist motor to the steering mechanism. With this manufacturing method, the outer peripheral side of the metal plate having the plate part to be mounted with a rotation transmitting system on a center side is press-formed circularly. With the press forming, the corrugated part is provided to have a plurality of concavities at one face of the metal plate and convexities to the other face side formed by said concavities which are arranged in a peripheral direction. The corrugated cylinder part is provided by bending said corrugated part to be a cylindrical form with respect to the plate part and a ring-shaped tooth body made of synthetic resin is integrally molded on the exterior side of said corrugated cylinder part.
With this method of manufacturing the gear, relatively long concavities of said corrugated part can be formed with a relatively small presser since the corrugated part is provided by performing press forming on the outer peripheral side of the metal plate having the plate part on the center side circularly, that is to say since the corrugated part configured as a corrugated plate is provided on the outer peripheral side of the metal plate. Consequently the method ensures an enough contact area between the tooth body and the support body, and enhances uniting strength between the tooth body and the support body. In addition, the corrugated cylinder part having relatively long concavities is provided simply since the corrugated cylinder part is provided by bending the corrugated part configured as a corrugated plate with respect to the plate part. Consequently the support body is formed with a relatively small presser, and manufacturing costs of the support body and the gear having said support body can be reduced all the more.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.